A constant challenge for lithography is to devise tools, materials, and processes that can reliably, efficiently, and quickly pattern structures with ever decreasing dimensions and pitch.
Directed Self-Assembly (DSA) lithography is a materials-based approach to lithography that relies on the phase separation of certain polymeric materials. A basic DSA process consists of coating a polymeric self-assembling material on a substrate over a previously formed guide (template) structure. The polymeric self-assembling material contains two (or more) components of different chemical nature. These components can for instance be blocks bonded together into a block copolymer. Immediately after coating, the blocks are relatively disordered, however, these blocks can be induced to separate into distinct phases and thereby form features with a characteristic pitch that is defined by the chemical composition of the block co-polymer. By coating such a block co-polymer film on a substrate with a well-defined guide structure, in terms of geometry and/or chemistry, the assembly of the block copolymer domains can be controlled. The pattern density achievable in the directed self-assembled block copolymer is typically a multiple of the pattern density of the guide structure. This increase in feature density, known as “density multiplication”, yields an assembled pattern with greater resolution than the original, lithographically defined chemical pattern.
In Chi-Chun Liu et al. (macromolecules 2011, 44, 1876-1885) and in Delgadillo et al., (Proc. Of SPIE Vol. 8680 86800L-1, 2013), a chemical pattern fabrication method is disclosed with precise control in chemistry and geometry utilizing specialized materials and lithographic patterning techniques. The process started with the formation of a cross-linked polymer mat (cross-linked polystyrene, X-PS) on a substrate, and the formation of a resist line pattern on the X-PS. The resist line pattern was subsequently trimmed with oxygen plasma and simultaneously the X-PS between the lines was etched. This exposed the substrate in desired locations, formed XPS guide lines, and decreased the width of the resist pattern lines. Hydroxyl-terminated polymer brush was then coated and grafted to the exposed substrate to fill the trenches between the X-PS lines. However, we observed that the resulting pre-pattern structure has unwanted characteristics: The side walls of the XPS guide lines had a sloped profile, the guiding strength of the structure may not be optimal, and the subsequent stripping of the resist lines required aggressive stripping chemistries which, in addition to imposing environmental, health and security constraints, also impact the top of the XPS guide lines (which acts as the pinning surface for one of the block co-polymer domains during DSA). There is therefore still a need in the art for good methods to form patterns for directed self-assembly.